It is now understood that processes such as re-encoding of a video signal can be significantly improved with knowledge of at least some of the coding decisions used in the original encoding.
Proposals have been made for making some or all of these coding decisions available explicitly. Examples of these proposals can be seen in previous patent applications [see EP 0 765 576 and EP 0 913 058]. These methods involve the use of an Information Bus which passes MPEG coding parameters from a decoder to a subsequent re-coder.
However, in certain situations, no such explicit information is available, and the only available information is that contained in the decoded video signal.
It will be well understood that in the MPEG-2 video compression standard, there are different categories of frames which differ in the degree to which their frames are coded using prediction, and that these categories are denoted by I-, P- and B-frames respectively. An important coding decision to be taken into consideration in a re-encoding process is accordingly the frame structure in terms of the I-, P- and B-frames.